doc: update BUILD.md
doc: update BUILD.md

the dep command isn't necessary now.

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@gmail.com>

fujita/gobgp,osrg/gobgp,a16/gobgp,j-kato/gobgp,tamihiro/gobgp,a16/gobgp,a16/gobgp,j-kato/gobgp,fujita/gobgp,a16/gobgp,a16/gobgp,osrg/gobgp,j-kato/gobgp,a16/gobgp,tamihiro/gobgp,fujita/gobgp,fujita/gobgp,tamihiro/gobgp,osrg/gobgp,osrg/gobgp,tamihiro/gobgp,a16/gobgp